Semiconductor wafers are generally prepared from a single crystal ingot (e.g., a silicon ingot) which is sliced into wafers. The wafers are ground and/or polished to produce product wafers. While reference will be made herein to semiconductor wafers constructed from silicon, other materials may be used as well, such as germanium or gallium arsenide.
One type of wafer is a silicon-on-insulator (SOI) wafer. An SOI wafer includes a thin layer of silicon atop an insulating layer (e.g., an oxide layer), which is in turn disposed on a silicon substrate.
An exemplary process of making an SOI wafer includes depositing a layer of oxide on a polished front surface of a donor wafer and/or a handle wafer. Particles (e.g., hydrogen atoms or a combination of hydrogen and helium atoms) are implanted at a specified depth beneath the front surface of the donor wafer. The implanted particles form a cleave plane in the donor wafer at the specified depth at which they were implanted. The surface of the donor wafer is cleaned to remove organic compounds deposited on the wafer during the implantation process.
The front surface of the donor wafer is then bonded to a handle wafer to form a bonded wafer pair through a hydrophilic bonding process. The donor wafer and handle wafer are bonded together by exposing the surfaces of the wafers to plasma containing, for example, oxygen or nitrogen. Exposure to the plasma modifies the structure of the surfaces in a process often referred to as surface activation. The wafers are then pressed together and a bond is formed therebetween. This bond is relatively weak, and must be strengthened before further processing can occur.
In some processes, the hydrophilic bond between the donor wafer and handle wafer (i.e., a bonded wafer) is strengthened by heating or annealing the bonded wafer pair at temperatures between approximately 300° C. and 500° C. The elevated temperatures cause the formation of covalent bonds between the adjoining surfaces of the donor wafer and the handle wafer, thus solidifying the bond between the donor wafer and the handle wafer. Concurrently with the heating or annealing of the bonded wafer, the particles earlier implanted in the donor wafer weaken the cleave plane. A portion of the donor wafer is then separated (i.e., cleaved) along the cleave plane from the bonded wafer pair to form the SOI wafer.
The bonded wafer is first placed in a fixture in which mechanical force is applied perpendicular to the opposing sides of the bonded wafer in order to pull a portion of the donor wafer apart from the bonded wafer. According to some methods, suction cups are utilized to apply the mechanical force. The separation of the portion of the donor wafer is initiated by applying a mechanical wedge at the edge of the bonded wafer at the interface between the donor wafer and the handle wafer. The application of the mechanical force initiates propagation of a cleave along the cleave plane. The mechanical force applied by the suction cups then pulls a portion of the donor wafer away from the bonded wafer, thus forming an SOI wafer.
The resulting SOI wafer thus comprises a thin layer of silicon disposed atop the oxide layer and the handle wafer. The thickness of the layer may be non-uniform. The layer may also have a non-uniform roughness. This non-uniform thickness and roughness of the layer may be the result of the cleave propagating at varying speeds and/or the mechanical force applied by the suction cups. Additional processing is thus required to reduce the variation in thickness of the layer and/or smooth this layer. These additional processing steps are both time consuming and costly.
Thus, there remains a continuing need for a system and method for cleaving a bonded wafer structure that results in the structure having a layer with a relatively uniform thickness and roughness.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.